For at least a decade scientists have attempted to attach electrodes or arrays of electrodes to tissue in order to monitor individual neurons in either the brain or nerves in order to electrically stimulate or sense neuron activity to treat nerve damaged maladies. What has been lacking is a relatively simple and rapid yet effective method of attaching an array of electrodes to tissue for monitoring neuron activity where the electrodes will remain reliably connected and functioning for an extended period of time to an active patient.
One major problem facing scientists in this endeavor is the fact that tissue such as the brain and nerves, has protective coverings that must be penetrated by the array of electrodes in order to accomplish the stimulating and monitoring function related to neurons at a desired site in the tissue. In order to accomplish successful penetration, the electrodes are mounted to a supporting platform and preferably have stiff and sharply pointed distal tips. For proper penetration it is considered necessary to impart sufficient impact on the supporting platform to cause an insertion speed of no less than 8.3 meters/second. Even with such insertion speeds, it will be recognized that without any anchoring mechanism, there will most likely be a disconnection between the electrode tips and the tissue during significant movement of the patient.
What is needed therefore, when considering long term stability, is an electrode that maintains electrical connection with the site selected for neuron monitoring and stimulation despite severe movements by the patient, thus being mechanically isolated from the platform while being in electrical communication with the platform.
The filament of an embodiment of the present invention is formed of a biocompatible stiffness enhanced pliable electrically conductive material which accommodates both the application of tissue stimulating electrical signals as well as the sensing of electrical signals emanating from the tissue. A contemplated application of the filament is in the field of neural prosthesis for the restoration of sensory and motor function and in particular in the design of a brain machine interface for neuroprosthetic control. The implications, practical engineering challenges and short comings of such machine interface design have been well documented over the recent past.
For example, U.S. Patent Application Publication No. 2007/0067007 published Mar. 22, 2007, and assigned to the same assignee hereof and incorporated by reference herein in its entirety discloses a new and novel hermetically sealed three dimensional electrode array contemplated for use in applications for neuron interface especially involving human nerves and the brain. The publication is also rich in identifying multiple references relating to the field of neural prosthesis.
As will be appreciated, the design and fabrication of the elements used to establish electrical contact with living tissue has been a daunting task. Electrode designs of the past have resulted in electrode arrays that have been rigid and typically of relatively large diameter. On the advantage side, such electrodes provide for reliable penetration and therefore contact with selected living tissue. On the disadvantage side the electrodes are not capable of individual movement in concert with moving tissue with which the electrodes are in contact. This is particularly important when considering permanently stiff and rigid electrodes used for implant in the human brain to detect sensory cortex signals and provide motor cortex stimulation signals.
Access to the motor and sensory cortex portions of the brain for neuron contact, especially with the use of miniaturized battery powered micro stimulators/sensors as described in U.S. Pat. No. 6,185,452, is particularly advantageous in accomplishing functional electrical stimulation in patients having interruptions in neuromuscular pathways, leading to severely physically compromised and handicapped patients.
As is widely understood, the brain is protected by the thick bones of the skull and is suspended in cerebrospinal fluid and as such is capable of relative movement in the fluid. It is estimated that the brain contains 50-100 billion neurons of which about 10 billion are cortical pyramidal cells which pass signals to each other via approximately 100 trillion synaptic connections. The signals are very site specific such that a signal from the sensory cortex relating for example, to the eyelid, will emanate from a particular site whether or not that site has experienced relative movement due to movement of the brain in the fluid. Movement of the brain may occur for example, when the patient jumps or runs or falls or moves his head in a rapid manner and the like.
Movement of adjacent portions of brain tissue also create significant problems in that although one electrode in an electrode array may consistently monitor signals from a pre-determined site, an adjacent electrode may lose consistency by monitoring other albeit adjacent tissue signals than from the intended site. Tissue deformation and injury is another potential problem, in that the rigidity of the electrodes which is required during implant to penetrate living tissue, functions as a rigid blade as the tissue moves below it potentially inflicting injury to the tissue.
Contemporary philosophy on the nature of the electrodes is described in a textbook entitled Neural Prosthesis for Restoration of Sensory and Motor Function, edited by John K. Chapin and Karen A. Moxon, published by CRC Press, ISBN 0-8493-2225-1.
As described in the referenced textbook, wires intended for neuroprosthetic devices to record from or stimulate the neural tissue, must have a small diameter while maintaining adequate stiffness and tip shape to penetrate the dura and traverse the tissue with minimal bending and mechanical disturbance to adjacent tissue. Benefits were enumerated in the use of 25-50 micron wires including being strong enough to go through the dura of certain animals and rigid enough to be lowered into the correct position. Deficiencies were also enumerated in the use of uncoated wire smaller than 25 microns in that penetrating the dura was difficult and it can be diverted by a fiber bundle when lowered into the brain and therefore, they often do not end up in the intended brain site. Thus electrodes formed of wire filaments in the range of about 10 microns to 25 microns would undoubtedly be beyond the scope of the prior art.
In order to provide an electrode not shown or suggested in the prior art, the electrode and in particular, the distal tip of the electrode which is in electrical contact with the tissue, must at one time be stiff and sufficiently rigid to accommodate penetration of the electrode into the dura and then the brain tissue, but then become or return to sufficient pliability and flexibility to move in concert with the moving tissue into which the electrode has penetrated without any movement away from the original site of penetration of the distal tip in the brain tissue. The filament of the present invention provides the above characteristics.